Heartworms

Heartworms

by Debra Offenbacker, D.V.M.

Heartworms, or Dirofilaria immitis, causes serious and potentially life-threatening cardiac disease in both dogs and cats. Heartworm infections have been diagnosed in both all around the world, including in all 50 states in the USA. This worm is transmitted to the animals by mosquitoes, and because infected mosquitoes can come inside your house, both outdoor and indoor pets are at risk. Adult heartworms are long and thin. Females can grow up to 10 inches while males only reach half that length. Although heartworms can live in the right ventricle of the heart, most reside in the pulmonary artery, the vessel that carries blood from the right side of the heart to the lungs. Female worms can live for several years, releasing millions of immature worms, called microfilariae, into the animal’s bloodstream. Microfilariae are so small they are sucked up by the mosquito as it takes a blood meal from your pet. The mosquito is an essential intermediate host for heartworms, meaning it can only mature by spending time in a mosquito. Once in a mosquito for the required amount of time, the larva leaves the mosquito when it takes a blood meal from its next victim. Once the larva is in a dog, it travels around and finally takes up residence in the blood vessels. It takes six months for the heartworm to become completely mature and reproductively viable. Heartworms cause an inflammatory response inside the pulmonary artery. This restricts blood flow, causing the heart muscles to have to work harder. Eventually, the strain can be so great that the heart fails. Dogs with heartworms usually suffer from a soft cough. Those with heavy infestations are also intolerant to exercise, lose weight, and have difficulty breathing. Cats with heartworm may cough, but they can also show some unexpected signs. In long-term infestations, they may vomit, breathe hard, and lose weight. In short-term infestations, cats may collapse, go blind, convulse or even die suddenly.

Heartworm in dogs is diagnosed with blood tests. The simplest method is to look for microfilaria on a blood smear under a microscope. Other tests detect the presence of adult female heartworm proteins called antigens in the blood. These are the most widely used tests in veterinary hospitals. However, at the time of diagnosis, your veterinarian may advise x-rays and an ultrasound of the heart (echocardiography) to assess the consequences of the disease on the heart. Treatment of heartworm has come a long way. Worms were originally removed from the heart by putting a worm hook down the jugular vein into the heart and then pulling out the worms one by one. This procedure was very risky, and because the majority of the worms were in the pulmonary artery where they couldn’t be reached, not very effective. Another technique that The American Heartworm Society no longer permits, is the use of preventatives being used in a long-term, slow-kill method. The American Heartworm Society recommends a multi-step program, using antibiotics and heartworm preventatives prior to the three-dose regiment of injectable medication which kills the adult worms (adulticide). The initial injection is followed a month later by two injections of the same medication, 24 hours apart. The problem is that dying worms can travel down the arteries, blocking them and causing embolisms. The inflammatory response to the dying/dead worms can make the dog quite ill. Treatment of heartworms is very expensive and time consuming, not to mention it puts our pets in danger.

It is much simpler to prevent heartworm disease from occurring than treating it afterwards, which is why preventative measures are essential. The American Heartworm Society recommends year-round heartworm prevention, such as Heartworm preventive tablets, which are given orally once every 30 days. Most heartworm medications have the added benefit of killing some of the intestinal worms too. There are also 30 day topical medications, as well as an injectable preventative that is continues to be effective for six months. Cats tend to have smaller burdens of heartworms consisting of only male worms or immature worms. The clinical importance of heartworms is greater in cats because even a small number of heartworms are potentially life threatening. The more common diagnostic testing used on dogs cannot be relied upon to rule out heartworm disease in cats. The preferred method for screening cats for heartworm disease includes using both antigen and antibody testing. Radiographs and echocardiography are also used when diagnosing cats with heartworm infection. If a cat is not showing any clinical signs it may be prudent to allow time for a spontaneous cure. Adulticide use is not recommended in cats however surgery is an option in heavily infected cats. Monthly heartworm preventives are a safe and effective option for cats, even the socalled “indoor” cat. If you would like to know more about heartworm disease in dogs and cats, please visit the American Heartworm Society website heartwormsociety.org